Stoke Audi were doing good deals on 5L Quantum Longlife 3 5W-30 Murdo - perhaps worth a call ?. I bought some delivered for under £30 - sorry i can,t help you with Castrol.
Ta
Dave
I used Petronas Syntium 5000 AV, 4 litres for £24.99 from ECP. Meets the VW extended interval specifications.